The field of in vitro diagnostics is quickly expanding as the need for systems that can rapidly detect the presence of harmful species or determine the genetic sequence of a region of interest is increasing exponentially. Current molecular diagnostics focus on the detection of biomarkers and include small molecule detection, immuno-based assays, and nucleic acid tests. The built-in specificity between two complementary nucleic acid strands allows for fast and specific recognition using unique DNA or RNA sequences, the simplicity of which makes a nucleic acid test an attractive prospect. Identification of bacterial and viral threat agents, genetically modified food products, and single nucleotide polymorphisms for disease management are only a few areas where the advancement of these molecular diagnostic tools becomes extremely advantageous. To meet these growing needs, nucleic acid amplification technologies have been developed and tailored to these needs of specificity and sensitivity.
Historically, the most common amplification technique is the polymerase chain reaction (PCR), which has in many cases become the gold standard for detection methods because of its reliability and specificity. This technique requires the cycling of temperatures to proceed through the steps of denaturation of the dsDNA, annealing of short oligonucleotide primers, and extension of the primer along the template by a thermostable polymerase. Though many new advances in engineering have successfully shortened these reaction times to 20-30 minutes, there is still a steep power requirement to meet the needs of these thermocycling units.
Various isothermal amplification techniques have been developed to circumvent the need for temperature cycling. From this demand, both DNA and RNA isothermal amplification technologies have emerged.
Transcription-Mediated Amplification (TMA) employs a reverse transcriptase with RNase activity, an RNA polymerase, and primers with a promoter sequence at the 5′ end. The reverse transcriptase synthesizes cDNA from the primer, degrades the RNA target, and synthesizes the second strand after the reverse primer binds. RNA polymerase then binds to the promoter region of the dsDNA and transcribes new RNA transcripts which can serve as templates for further reverse transcription. The reaction can produce a billion fold amplification in 20-30 minutes. This system is not as robust as other DNA amplification techniques and is therefore, not a field-deployable test due to the ubiquitous presence of RNAases outside of a sterile laboratory. This amplification technique is very similar to Self-Sustained Sequence Replication (3SR) and Nucleic Acid Sequence Based Amplification (NASBA), but varies in the enzymes employed.
Single Primer Isothermal Amplification (SPIA) also involves multiple polymerases and RNaseH. First, a reverse transcriptase extends a chimeric primer along an RNA target. RNaseH degrades the RNA target and allows a DNA polymerase to synthesize the second strand of cDNA. RNaseH then degrades a portion of the chimeric primer to release a portion of the cDNA and open a binding site for the next chimeric primer to bind and the amplification process proceeds through the cycle again. The linear amplification system can amplify very low levels of RNA target in roughly 3.5 hrs.
The Q-Beta replicase system is a probe amplification method. A probe region complementary to the target of choice is inserted into MDV-1 RNA, a naturally occurring template for Q-Beta replicase. Q-Beta replicates the MDV-1 plasmid so that the synthesized product is itself a template for Q-Beta replicase, resulting in exponential amplification as long as the there is excess replicase to template. Because the Q-Beta replication process is so sensitive and can amplify whether the target is present or not, multiple wash steps are required to purge the sample of non-specifically bound replication plasmids. The exponential amplification takes approximately 30 minutes; however, the total time including all wash steps is approximately 4 hours.
Numerous isothermal DNA amplification technologies have been developed as well. Rolling circle amplification (RCA) was developed based on the natural replication of plasmids and viruses. A primer extends along a circular template resulting in the synthesis of a single-stranded tandem repeat. Capture, washing, and ligation steps are necessary to preferentially circularize the template in the presence of target and reduce background amplification. Ramification amplification (RAM) adds cascading primers for additional geometric amplification. This technique involves amplification of non-specifically sized strands that are either double or single-stranded.
Helicase-dependent amplification (HDA) takes advantage of a thermostable helicase (Tte-UvrD) to unwind dsDNA to create single-strands that are then available for hybridization and extension of primers by polymerase. The thermostable HDA method does not require the accessory proteins that the non-thermostable HDA requires. The reaction can be performed at a single temperature, though an initial heat denaturation to bind the primers generates more product. Reaction times are reported to be over 1 hour to amplify products 70-120 base pairs in length.
Loop mediated amplification (LAMP) is a sensitive and specific isothermal amplification method that employs a thermostable polymerase with strand displacement capabilities and four or more primers. The primers are designed to anneal consecutively along the target in the forward and reverse direction. Extension of the outer primers displaces the extended inner primers to release single strands. Each primer is designed to have hairpin ends that, once displaced, snap into a hairpin to facilitate self-priming and further polymerase extension. Additional loop primers can decrease the amplification time, but complicates the reaction mixture. Overall, LAMP is a difficult amplification method to multiplex, that is, to amplify more than one target sequence at a time, although it is reported to be extremely specific due to the multiple primers that must anneal to the target to further the amplification process. Though the reaction proceeds under isothermal conditions, an initial heat denaturation step is required for double-stranded targets. Amplification proceeds in 25 to 50 minutes and yields a ladder pattern of various length products.
Strand displacement amplification (SDA) was developed by Walker et. al. in 1992. This amplification method uses two sets of primers, a strand displacing polymerase, and a restriction endonuclease. The bumper primers serve to displace the initially extended primers to create a single-strand for the next primer to bind. A restriction site is present in the 5′ region of the primer. Thiol-modified nucleotides are incorporated into the synthesized products to inhibit cleavage of the synthesized strand. This modification creates a nick site on the primer side of the strand, which the polymerase can extend. This approach requires an initial heat denaturation step for double-stranded targets. The reaction is then run at a temperature below the melting temperature of the double-stranded target region. Products 60 to 100 bases in length are usually amplified in 30-45 minutes using this method.
These and other amplification methods are discussed in, for example, VanNess, J, et al., PNAS 2003. vol 100, no 8, p 4504-4509; Tan, E., et al., Anal. Chem. 2005, 77, 7984-7992; Lizard, P., et al., Nature Biotech. 1998, 6, 1197-1202; Notomi, T., et al., NAR 2000, 28, 12, e63; and Kurn, N., et al., Clin. Chem. 2005, 51:10, 1973-1981. Other references for these general amplification techniques include, for example, U.S. Pat. Nos. 7,112,423; 5,455,166; 5,712,124; 5,744,311; 5,916,779; 5,556,751; 5,733,733; 5,834,202; 5,354,668; 5,591,609; 5,614,389; 5,942,391; and U.S. patent publication numbers US20030082590; US20030138800; US20040058378; and US20060154286.
There is a need for a quicker method of amplification of single-stranded and double-stranded nucleic acid target sequences that can be performed without temperature cycling and that is suitable for shorter target sequences.